Building block



FIG. 1 is a front perspective view of my ornamental design for a building block;

FIG. 2 is a rear perspective view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a right side view thereof, the left side view is the same;

FIG. 5 is a top view thereof; and,

FIG. 6 is a bottom view thereof. 

The ornamental design for a building block, as shown and described. 